N° 24–2000: Ulysses feels the brush of a comet's tail

6 April 2000

A chance encounter between Ulysses, the joint ESA/NASA spacecraft, and a comet's tail is leading scientists to conclude that comet tails extend much further than anyone supposed - right to the edge of the solar system. Two papers published in Nature today report that on 1 May 1996, Ulysses flew through the tail of comet Hyakutake whose nucleus was more than 3.5AU (one AU equals the Sun-Earth distance) away at the time. "This makes it the longest comet tail ever recorded", says Geraint Jones from Imperial College, London who is a member of one of the two instrument teams that made the discovery.

"Ulysses's prime task is to map the solar wind above the Sun's poles: it had not been looking for Hyakutake, which happened to be at its closest approach to the Sun on 1 May 1996, or any other comet", says Richard Marsden, ESA's Ulysses Project Scientist. "Ulysses was just in the right place at the right time."

The two teams stumbled across the telltale signature of a comet quite independently when poring over old Ulysses data. Jones and colleagues found their evidence in magnetic field data: "the magnetic field lines were draped in a way that you'd expect in a comet's tail," says Jones. The other instrument team, lead by George Gloeckler from the University of Maryland, found their evidence when looking at the composition of the solar wind. Cometary tails are rich in oxygen and carbon compared with the solar wind, but depleted in nitrogen and neon.

The Imperial College team identified Hyakutake as the source of the anomalous readings. On 1 May 1996, Ulysses was aligned with the Sun and the position Hyakutake had occupied eight days earlier, which Jones calculated was the time needed for material leaving the comet's nucleus to travel the distance to Ulysses.

One of the most surprising aspects of the discovery is the length of Hyakutake's tail. Cometary experts had thought that comet tails eventually spread out and lose their integrity. "We found that the whole thing is preserved as an entity and doesn't spread out very much," says Gloeckler. "If it can persist as far as Ulysses, there's no reason to presume that it wouldn't continue to the edge of the heliosphere (the boundary about 100AU from the Sun between the solar wind and the interstellar medium)," says Jones.

"This discovery makes us wonder whether Ulysses or other spacecraft have crossed a comet tail before. So we're going back to look again for other signatures. But it's probably a rare event," says Jones. The comet nucleus has to be in exactly the right position with respect to the Sun and the spacecraft for the tail to pass over the spacecraft at the right time - and the chances of that happening very often are probably small.

For more information please contact:

ESA - Communication Department

Media Relations Office

Tel: +33(0)1.53.69.7155

Fax: +33(0)1.53.69.7690

For information on Ulysses science:

Dr. Richard Marsden, Ulysses Project Scientist

ESA - Estec (The Netherlands)

Tel: +31 71 565 3583

Email: rmarsden@estec.esa.nl

For magnetometers results:

Dr. Geraint Jones

Space and Atmospheric Physics Group,

The Blackett Laboratory, Imperial College, London SW7 2BW, UK

Tel: +44 (0)20 7594 7774

Fax: +44 (0)20 7594 7772

Email: g.h.jones@ic.ac.uk

Dr. Tim Horbury

Astronomy Unit, School of Mathematical Sciences

Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK

Tel: +44 (0)20 7882 3181

Fax: +44 (0)20 8983 3522

Email: t.horbury@qmw.ac.uk

For SWICs results:

Dr. George Gloeckler,

Institute for Physical Science and Technology,

University of Maryland, College Park, MD 20742, USA

Tel: +1 301 405 6199

Fax: +1 301 314 9547

Email: gg10@umail.umd.edu

Dr. Johannes Geiss,

International Space Science Institute,

Hallerstrasse 6, CH-3012 Bern, Switzerland

Tel: +41 31 631 4892

Fax: +41 31 631 4897

Email: geiss@phim.unibe.ch

For further information on Ulysses visit the ESA science web page at:http://sci.esa.int/ulysses For further information on ESA visit http://www.esa.int